Memory devices are used in electronic systems and computers to store information in the form of binary data. These memory devices may be characterized as either volatile memory, where the stored data is lost if the power source is disconnected or removed or non-volatile, where the stored data is retained even during power interruption. An example of a non-volatile memory device is the magnetic random access memory (MRAM).
An MRAM can be formed, to considerable advantage, based on the storage of digital bits as alternative states of magnetization of magnetic materials in each memory cell, typically thin-film materials. These films may be thin magnetic films having information stored therein based on the direction of the magnetization occurring in those films. The information is typically obtained either by inductive sensing to determine the magnetization state, or by magnetoresistive sensing of each state.
An arrangement for sensing states of magnetization in thin film magnetic material portions used in memory cells for storing bits is based on choosing a thin film magnetic material which also exhibits a sufficient magnetoresistance property. Changes in electrical resistance of such a material with the application, removal or change in magnitude of a magnetic field do not depend on the dimensions of the film portion. Thus the film portion to store a bit can be made very small to improve the packing density of cells in a magnetic memory.
Such magnetic thin-film memories may be conveniently provided on the surface of a monolithic integrated circuit to provide easy electrical interconnection between the memory cells and the memory operating circuitry on the monolithic integrated circuit. When so provided, it is desirable to reduce the size and increase the packing density of the magnetic thin-film memory cells to achieve a significant density of stored digital bits.
Typically, a thin-film magnetic memory includes a number of bit lines intersected by a number of word lines. At each intersection, a thin film of magnetically coercive material is interposed between the corresponding word line and bit line. The magnetic material at each intersection forms a magnetic memory cell in which a bit of information is stored.
The word lines are often provided on a first metal interconnect layer and the bit lines are provided on another. In each case, the metal interconnect layers must typically be connected to supporting circuitry or other underlayer structures on the monolithic integrated circuit for the memory to function. In addition, portions of the first metal interconnect layer are often connected to portions of the second metal interconnect layer to complete selected circuit elements.
The number of metal interconnect layers, typically copper interconnect layers, in the proceeding arrangement requires two metal interconnects for each plane (or level) of magnetic memory cells. As density becomes an issue, the amount of substrate space required must be taken into account. The structural arrangement of a magnetoresistive memory device is a significant focus of the present invention.